The gemara on amud alef has three answers to explain the mishna. Tosafos writes that according to the first answer of the gemara that the mishna is speaking in a place where they call out so it is permitted to purchase from a goy, the mishna would contradict the braisa. To reconcile the contradiction Tosafos is forced to distinguish between a sale and a gift - only by a sale where money is being paid is there an issur of g'neivas da'as. However, according to the second and third answer Tosafos says that only when you explicitly lie is it considered g'neivas da'as, but if you just do something where the other party draws his own conclusion it would be considered איהו הוא דקא מטעי נפשיה (as we see in the gemara and isn't considered gneivas da'as). Tosafos modifies this slightly by saying when one does an action that is suggestive such as opening up a barrel of wine in the presence of the guest indicating that it is in his honor, that is tantamount to actually saying that you are doing it for him. Rashi seems to maintain that once the gemara introduces the concept of איהו הוא דקא מטעי נפשיה, we limit ALL the cases of geneivas da'as to where he explicitly told the person that he is doing it for him, or explicitly told the person that it was shechted meat. We see from here that one is not obligated to correct his friends misunderstanding of the situation, but cannot actively mislead him.